The present invention relates to the telephony art and to a method of communication using one access, in particular one integrated services digital network (ISDN) access. The invention aims more particularly to improve the organization of the access used for communication between exchanges, in particular private automatic branch exchanges (PABX). An object of the invention is to improve access operating conditions.
The ISDN standard covers various kinds of access, as is well known in the telephony art. For example, a primary rate access, referred to as a T2 access in the ISDN standard, can convey digital data at a bit rate of 2 Mbit/s. The bit rate can be organized by dividing it into 32 telephone paths, each of which can be routed at 64 kbit/s, for example. A T2 access normally provides thirty data paths, one signaling path and one maintenance path. The remainder of this description uses the terms path and channel. A path is a physical and functional medium that connects two parties. A channel is the physical use of the whole or part of one or more paths to convey messages on a telephone link between two or more parties.
Signaling is the information needed by a public network operator or by private exchanges to switch various transmission paths progressively set up along a transmission route to enable one party to be connected to another party. For example, a call set up message sent at call set up time constitutes signaling. The set up message naturally includes the address (in practice the telephone number) of the called party and, nowadays, increasing volumes of additional information, for example the caller's name and/or telephone number and service information. The service information is essentially characteristic information which relates to the nature of the telephone used to initiate the call and is used to give the calling telephone rights over the remote telephone once they are connected, for example the right to access voicemail, automatic entry of a password, etc.
A prior art access distributes the various paths on a common physical medium. In the case of the T2 access, for example, a bundle of identical resources is devoted to an access. On a very high bit rate (140 Mbit/s) channel, the access is an allocated frequency band and/or an identified time slot capable of conveying the 2 Mbit/s bit rate made available. On a physical medium, an adequate number of telephone pairs is used to route the information. With this kind of access, there is no communication if the physical medium is down: no data, no signaling messages, no maintenance messages.
With ongoing improvements in telecommunications, it has become necessary for the various channels used (B channels for voice and data or D channels for signaling and data) to be implemented on different physical media. For example, some of these media can be analog lines and others can conform to the Internet Protocol (IP) format or the X.25 format. Note in this case the independence of the telephone paths or channels. An access of this kind is referred to as a separate signaling access or hybrid access.
However, the problem presented by the signaling path is that no further calls can be set up should it fail. Nevertheless, the precise aim of diversifying the media is to avoid massive blocking of a connection between two exchanges. The result is therefore the opposite of what is required: the access is made less resistant to breakdowns, whereas the aim was precisely to make it more reliable.